The use of light metals such as aluminum (Al) alloy and high-strength steel sheets for automobile members has been promoted for the purpose of reducing weight in order to improve automobile fuel consumption. The light metals such as Al alloy offer the advantage of high specific strength. However, since such metals are much more costly than steel, their applications are limited to special applications. Thus, there is a need to increase the strength of steel sheet to promote cost decreases and automobile weight reductions over a wider range.
Since increasing the strength of a material typically causes deterioration of moldability (processability) and other material characteristics, one factor to developing high-strength steel sheet is the extent to which strength can be increased without deteriorating material characteristics. Since characteristics such as burring formability, ductility, fatigue durability and corrosion resistance may be important characteristics for steel sheet that are used for inner plate members, structural members and underbody members, and how effectively these characteristics can be balanced with high strength on a high order is important.
For example, Japanese Unexamined Patent Application, First Publication Nos. 2000-169935 and 2000-169936 describe transformation induced plasticity (TRIP) steel in which moldability (ductility and deep drawability) are improved. This may be due to the occurrence of TRIP phenomenon during molding by containing residual austenite in the microstructure of the steel in order to achieve both high strength and various advantageous characteristics, e.g., moldability.
Steel sheet obtained in this area can demonstrate a breaking elongation in excess of 35%, and superior deep drawability (limiting drawing ratio (LDR)) due to the occurrence of TRIP phenomenon by the residual austenite at a strength level of about 590 MPa. However, amounts of elements such as C, Si and Mn should preferably be reduced to obtain steel sheet having strength within the range of 370 to 540 Mpa. When the amounts of elements such as C, Si and Mn are reduced to realize the strength within the range of 370 to 540 MPa, there may be a problem of being unable to maintain amount of residual austenite required for obtaining TRIP phenomenon in the microstructure at room temperature. Thus, it may be difficult to apply high-strength steel sheet having strength of 540 MPa or higher to a member in which steel sheet having strength on the order of 270 to 340 MPa is currently used, without first improving operations and equipment used during pressing. One general known solution is to use steel sheet having strength of about 370 to 490 MPa. On the other hand, a preference for a reduction of gauges is ever increasing to achieve reduction in weight for automobile body. It is therefore important for the reduction in weight for automobile body to maintain pressed product strength, based on the premise of reducing gauges.
Bake-hardening (BH) steel sheet has been described as possibly solving these problems because it has low strength during press molding, and can improve the strength of pressed products as a result of introducing stress due to pressing and subsequent baking finish treatment.
It may be effective to increase solute C and solute N so as to improve bake hardenability. However, increases in these solute elements present in the solid solution can worsen aging deterioration at normal temperatures. Consequently, it is possible to develop a technology that can allow both bake hardenability and resistance to aging deterioration at normal temperatures.
Japanese Patent Application Nos. H09-278697 and 2000-028141 describe technologies for realizing both bake hardenability and resistance to aging deterioration at normal temperatures, in which bake hardenability is improved by increasing the amount of solute N, and the diffusion of solute C and solute N at normal temperatures is inhibited by an effect of increasing grain boundary surface area caused by grain refining of crystal grains.
However, the use of finer crystal grains may have the risk of leading to increases in the yield point and causing deterioration of press moldability. In addition, increasing the amount of solute N can have the advantage of increasing the BH amount. However, there is concern over considerable decreases in the BH amount after aging due to the appearance of yield point elongation caused by aging.